Yeah, in my area, we were quoted ~$1B for a train line extension of like 10-20 miles, on largely existing track. And that's wasn't high speed rail, but light rail.

Trains are expensive. Roads are expensive ($1.5-7.7M/mile according to this site). These costs also don't include ongoing maintenance.

Trains usually cost more in the short term, and cost less longer term. So $12B today is expensive, but it also means we don't need expensive expansions to i-15 and can easily increase people moved by increasing the frequency of train runs.